The discovery of high-temperature superconductors, commonly called high-T.sub.c superconductors, promises many technological applications. Although many types of low-T.sub.c superconducting devices are known, it has been difficult to fabricate high-T.sub.c equivalents. The highly anistropic properties and complex perovskite crystal structure of most high-T.sub.c superconductors have contributed to this difficulty. This following discussion will use YBa.sub.2 Cu.sub.3 O.sub.7-x, hereinafter called YBCO, as an example since it appears to be the most understood as well as technologically usable high-T.sub.c superconductor.
The crystal structure YBCO, illustrated in FIG. 1, is very closely related to the perovskite crystal structure, namely, an orthorhombic (or rectangular) structure with nearly equal a- and b-axes but a significantly longer c-axis. Unit cell parameters for YBCO, as well as those for SrTiO.sub.3, a popular substrate for growing YBCO thin films, are given in Table 1.
TABLE 1 ______________________________________ Lattice Parameters (nm) a b c ______________________________________ YBCO 0.382 0.388 1.168 PrBCO 0.387 0.393 1.171 SrTiO 0.391 0.391 0.391 ______________________________________
Arranged perpendicularly to the c-axis are planes of CuO.sub.2, which are believed to be largely responsible for the superconductivity. The perovskite crystal structure if described by A. F. Wells in his book entitled "Structural Inorganic Chemistry," 4th ed., Clarendon Press, 1975 at pages 149-154.
The elongated and non-symmetric unit cell has caused difficulty in growing monocrystalline samples of YBCO. Nonetheless, many techniques have found increasing success. One of the most successful has been pulsed laser deposition in the growth of YBCO thin films. Details of the technique have been disclosed by Venkatesan et al. in U.S. patent application, Ser. No. 07/331,795, filed Apr. 3, 1989, now U.S. Pat. No. 5,015,492, issued May 14, 1991, and by Hegde et al. In U.S. patent application, Ser. No. 07/360,909, filed Jun. 1, 1989, now U.S. Pat. No. 5,087,605, issued Feb. 11, 1992. In this technique, an ultraviolet laser delivers short light pulses onto the surface of a composite target having a composition similar to that of the desired film. The plume of the non-equilibrium evaporation from the target falls on the substrate and forms the desired YBCO film. With careful control of the substrate temperature and ambient oxygen pressure, monocrystalline films can be grown on (001) oriented substrates of SrTiO.sub.3, MgO, and other materials. Such oriented substrates have surface unit cells closely matching the a- and b-axis lattice parameters of YBCO. The YBCO films grown by this method had a c-axis orientation perpendicular to the substrate. These films have demonstrated transition temperatures T.sub.c in the range of 90.degree. C.
Hegde et al. further used this c-axis epitaxial growth to form an epitaxial heterostructure of YBCO--PrBCO--YBCO. The middle layer was PrBa.sub.2 Cu.sub.3 O.sub.7-y, which is also a perovskite but is non-superconducting and behaves like a semiconductor. Its lattice parameters are also given in Table 1. Their intent was to form a Josephson weak link across the PrBCO, that is, between the YBCO layers across the PrBCO layer. Josephson devices produced using this method reproducibly demonstrated S--N--S (superconductor--normal-metal--superconductor) behavior in their current-voltage characteristics. Both DC and AC Josephson effects were observed. However, the performance of such c-axis grown heteroepitaxial devices is limited by the very short coherence length along this crystallographic direction. The coherence length, one of the key length parameters in superconductivity, is highly anisotropic in YBCO, as well as other known high-T.sub.c superconductors. The coherence length has a value .xi..sub.c .apprxeq.0.7 nm along the c-direction and a value .xi..sub.a,b .apprxeq.3.5 nm within the a-b plane of YBCO. As a result, the coherence length in c-axis oriented YBCO--PrBCO-YBCO heterostructures is smaller than the smallest spacing that can be ideally achieved, specifically, a single unit-cell layer of PrBCO separating two layers of YBCO. It thus appears that even with ideal interfaces between YBCO and PrBCO, the performance of such c-axis oriented junctions will be limited.
On the other hand, the coherence length .xi..sub.a,b of .about.3.5 nm along the a- and b-directions of YBCO is nearly ten times the corresponding lattice parameters of PrBCO and YBCO. If heteroepitaxial structures could be grown with the c-axis lying within the film plane, they would provide a much wider latitude in the quality of the PrBCO barrier and of the YBCO--PrBCO interfaces. Even if the PrBCO barrier would be a few unit cells thick in the a,b-directions, there would still be sufficient overlap of the order parameters of the two YBCO electrodes.
In view of these potential advantages, many groups have attempted to grow a,b-axis oriented YBCO films. By a,b-axis oriented films is meant a film in which nearly all the material has its c-axis lying in the plane of the film. One approach to growing a,b-axis oriented perovskite films uses (110)-oriented substrates. A second approach simply lowers the substrate growth temperature by about 100.degree. C. At the lower temperatures, even on a (001) substrate, a large fraction of the YBCO forms in an a,b-axis orientation although there is some mixture with c-axis orientation.
Problems arise with both these approaches. In either case, the films back homogeneity in their orientation along a particular direction. X-ray diffraction has shown that for both these processes, it is difficult to obtain films which are purely a-axis oriented or b-axis oriented. Other phases are almost always locked in. Furthermore, both processes produce a poorly crystalline structure. Regardless of orientation, the crystalline structure must be highly ordered to permit the epitaxial growth of overlayers, such as the PrBCO barrier or the YBCO counter-electrode. So far, there has been no convincing demonstration of epitaxial growth on top of a,b-axis oriented films previously grown on (110) or (001) substrates at the lower deposition temperatures. Finally, the temperature range in which a,b-axis oriented growth is induced on these substrates is usually below the optimal deposition temperature required to obtain the .about.90.degree. C transition temperature achievable in c-axis oriented YBCO. Thus, efforts have failed in obtaining a,b-axis oriented films exhibiting good crystalline structure with a T.sub.c above the 60.degree.-70.degree. K range.
It would be further preferred to achieve a,b-axis oriented growth on (001) substrates, which to date have yielded the best c-axis oriented films. Thereby, a commonly oriented substrate would provide greater flexibility in designing devices incorporating films and heterostructures of both orientations.
The problem of a short c-axis coherence length is present, as well, in the bismuth and thallium superconductors represented by the formula A.sub.2 B.sub.2 Ca.sub.2n-1 Cu.sub.n O.sub.y, where A is either Bi or Tl, B is either Ba or Sr, n=1, 2, or 3, and y is between 6 and 10. These perovskite materials are of technological interest because they manifest significantly higher values of T.sub.c ; however, their c-axis lattice parameters are also significantly longer.